Apparatus and method for alleviation of symptoms by application of tinted light

ABSTRACT

Apparatus and a corresponding method for the diagnosis and alleviation of symptoms of visually induced physiological defects and/or pathological conditions is provided. A plurality of narrow-band light sources are combined to constitute a colour controllable lamp. A method for adjusting the settings of this lamp permits the optimum illumination for a particular subject to be found, whilst the latter carries out a task such as reading or writing. By use of the lamp to simulate the expected visual stimulus, to which the subject would be exposed if provided with viewing aids such as tinted spectacles and the like, an optimal selection from a database of such aids may be made or a new formulation defined. Inter alia, the symptoms of visual dyslexia, macular degeneration and visually induced migraine may be alleviated.

[0001] The current invention is concerned with the provision orfiltration of the illumination for a given task, such as reading orwriting, and, specifically, with helping to alleviate the symptoms ofcertain physiological defects, such as dyslexia, or pathologicalconditions, such as migraine or macular degeneration, which may besuffered by the subject undertaking the task.

BACKGROUND

[0002] It is known that the response of the visual system is affected bythe stimuli, which it receives. The threshold for such stimulationvaries between individuals and, under adverse conditions, cansignificantly reduce performance. When the visual system is overstimulated, it reacts in a number of ways. Amongst a variety ofundesirable effects, which can be caused, two examples include a drop inconvergence sufficiency and a reduction in the ability to accommodate orfuse images. In addition, visual dyslexia may become apparent andmigraines can be caused. Visual dyslexia is a condition of impairedreading and writing ability due to visual perception or visualisationproblems. It is apparent therefore that for some it is necessary tomodify the visual stimulus by changing the spectral distribution in aspecific task e.g. reading and writing in school. In summary, it is wellestablished that the colour of ambient lighting has a major influence onthe effects of disorders such as dyslexia, epilepsy and migraine. In thecase of dyslexia some sufferers can alleviate their reading problems bycovering the page with a transparent coloured overlay in order to blockout those wavelengths of light which give rise to an aspect of theirproblem. These overlays typically remove various amounts of simpleprimary colours, such as red, green or blue light and whilst they mayassist with reading, they are of no value for writing.

[0003] In U.S. Pat. No. 5,855,428 (Wilkins) apparatus is described inwhich the spectral distribution of light from a fluorescent lamp toilluminate a surface to support reading material is altered by theinterposition of specifically selected broadband filters. By adjustmentof the position of the selected filter or filters different colours andsaturation thereof can be selected.

[0004] In U.S. patent application Ser. No 2001/0005319 A1 (Ohishi etal.) an illumination control system, for general use, is described, inwhich the coordinates in colour space of the controlled illumination arearranged to follow a predetermined locus of points by mixing specificamounts of light from a plurality of differently coloured light emittingdiodes (LED's).

[0005] Neither of these documents identifies the benefit of usingsources which are characterised by providing light with a spectraldistribution which is relatively narrow for application to thealleviation of the symptoms of the physiological defects and/orpathological conditions identified herein. This would be the case forlaser sources, super-luminescent LED's and conventional coloured LED's,which provide light with a typical spectral bandwidth of between 17 nmto around 50 nm. The provision of illumination using additive lightsources, such as LED's for the quantitative diagnosis and alleviation ofthe symptoms identified is the subject of this invention.

SUMMARY OF THE INVENTION

[0006] It is an object of the current invention to provide optimalillumination for a subject who may be suffering from physiologicaldefects or pathological conditions of his/her visual system in order toalleviate the symptoms thereof.

[0007] It is a further object of the current invention to provide ameans for specifying a colour formulation for the lenses of thespectacles to be worn by a patient suffering from one or more of theaforesaid physiological defects or pathological conditions.

[0008] Using a specific controllable light source for a particular taskcan be preferable to other forms of treatment (e.g. tinted spectacles),as the task lighting can be tailored precisely, for example to takeaccount of the ambient conditions. A specific light is also ofparticular importance in certain eye conditions such as maculardegeneration or cataract as optimum performance is directly related tovisual stimulus input, particularly if the person has relatively poorvision. Specific stimulus modification will also be of great use inmigraine prevention and treatment with possible uses in attentiondeficit hyperactivity syndrome and some types of epilepsy. Where it isdesirable for the subject to use tinted spectacles, a controllable lightsource, as described herein, is a useful tool for defining the preferredfilter characteristics of the tinted lenses.

[0009] Thus, according to one aspect of this invention means is providedfor the quantitative diagnosis and/or alleviation of the symptoms of aplurality of visually induced physiological defects and/or pathologicalconditions suffered by a subject comprising a plurality of lightsources, each of which is arranged to emit a respective spectralcomponent of the visible spectrum, and control means for selecting aweighted mixture of said spectral components to provide illumination,characterised in that, in use, said illumination is arranged toilluminate a surface for viewing by the subject; said mixture is anadditive combination of the spectral components emitted by at least twoof said light sources; and the control means provides the means forvarying the amount of illumination from each of said at least two lightsources to impinge on said surface whereby, in use, a combination ofsaid spectral components is provided to alleviate the symptoms of atleast one of said visually induced physiological defects and/orpathological conditions.

[0010] The physiological defect may be visual dyslexia, visually inducedmigraine or macular degeneration.

[0011] Preferably a spectral component has a dominant wavelength whichcontributes to a respective first tristimulus value of the lightentering an eye of the subject whilst substantially maximising the ratioof said contribution to said first tristimulus value to the root meansquare of the contributions by said dominant wavelength to each of thesecond and third tristimulus values of the light entering the eye of thesubject.

[0012] Advantageously, a first spectral component may comprise adominant wavelength located between 465 nm and 475 nm. Another spectralcomponent may comprise a dominant wavelength located between 520 nm and530 nm. A third spectral component may have a dominant wavelength in therange 610 nm and 650 nm.

[0013] In preferred embodiments of the invention each light sourcecomprises at least one light emitting diode arranged to provide one ofthe spectral components. Preferably a spectral component has a spectralpower distribution having a width at half height which does not exceed50 nm.

[0014] Advantageously, the illumination from each of the light sourcesis diffused prior to impinging on the viewed surface so that therelative intensity of the light impinging at two points spaced on saidsurface is substantially the same for each of said spectral components.

[0015] According to a further aspect of the invention means is providedfor computing the combined effect of at least two of the activeillumination spectra, the ambient illumination spectrum, the reflectancespectrum of the target or an illuminated surface, the transmissionspectrum of at least one filter and the transmission spectrum of asurface coating over the visible spectrum, so that in use, the subject'sretinal response may be predicted and the settings of the active lightsource and/or the formulation of a filter to be used by the subject maybe optimised.

[0016] According to another aspect of the invention, a method for thediagnosis of a plurality of visually induced physiological defectsand/or pathological conditions suffered by a subject comprises arranginga plurality of light sources to emit different spectral componentswithin the visible spectrum, characterised by implementing thesuccessive steps of:—

[0017] (a) assessing the subject's performance with a series of targetsunder different levels of each of a plurality of illuminants, comprisingindividual spectral components or pre-determined ratios thereof,

[0018] (b) recording the optimum level (or at least the level at whichthe subject's performance improves) of each of said illuminants and

[0019] (c) combining the levels of each respective illuminant asrecorded in step (b) to provide a resultant additive mix of saidilluminants.

[0020] Preferably, the method includes the further step of applyingvariations to the level of each of the spectral components in smallsteps whilst combined in order to establish the mix of said illuminantswhich substantially improves or optimises the subject's performance.

[0021] According to yet another aspect of the invention a method forsimulating the performance of a selected filter comprises the steps of:—

[0022] (a) defining the tristimulus values of the tint which would beobserved by a subject when said filter is used in transmission forviewing a reading surface (or other reflective or transmissive surface);

[0023] (b) providing a colour controllable lamp comprising narrow bandcoloured light sources;

[0024] (c) illuminating the reading surface (or other reflective ortransmissive surface) for viewing by the subject with said lamp and

[0025] (d) selecting the level of illumination provided by each lightsource so that, in use, the defined tristimulus values are observed bythe subject.

[0026] Preferably the method includes the step of simulating a range ofpre-formulated filters and lighting conditions, whereby the subject canselect one or more filters for use under said conditions. and the methodincludes the further step of formulating and/or selecting the filter tooptimise the subject's performance.

[0027] The invention permits the formulation of filters with or withoutanti-reflection coatings for spectacles, contact lenses, colouredoverlays or any other tinted material a purpose of which is to alleviateproblems caused by colour related disorders of the human visual system.

[0028] Further scope of applicability of the present invention willbecome apparent from the detailed description given hereinafter.However, it should be understood that the detailed description andspecific examples, while indicating preferred embodiments of theinvention, are given by way of illustration only, since various changesand modifications within the spirit and scope of the invention willbecome apparent to those skilled in the art from this detaileddescription.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0029] The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawings,which are given by way of illustration only, and thus are not limitativeof the present invention.

[0030] The invention will now be described with reference to FIGS. 1a to7 in which:—

[0031]FIG. 1a illustrates the response of the human visual system, as afunction of the wavelength of the light incident thereon. Additionalcurves are provided to aid in the description of the invention.

[0032]FIG. 1b provides further curves showing the sensitivitycharacteristics of the colour receptors or cones at the human retina.

[0033]FIG. 2 illustrates, diagrammatically, apparatus constructed inaccordance with the invention in order to provide a colour controllablesource of illumination,

[0034]FIG. 3 shows in flowchart form a preferred method in accordancewith the invention for use of the apparatus of FIG. 2.

[0035]FIG. 4 shows the transmission spectrum of a typically tinted lens,formulated to reduce the relative stimulus to one type of cone, inaccordance with the invention.

[0036]FIG. 5 provides a graphical illustration of colour space and theposition of the colour co-ordinates of sources, as used in embodimentsof the invention, within this.

[0037]FIG. 6 illustrates an alternative embodiment of apparatusconstructed in accordance with the invention.

[0038]FIG. 7 illustrates a further embodiment of apparatus constructedin accordance with the invention.

[0039]FIG. 1a shows the so-called spectral tristimulus values as afunction of wavelength λ. These curves, which represent the amounts ofidealised primaries required to match any of the pure spectral coloursin the visible range and are related to the colour sensitivitycharacteristics of the human eye. Curve 1, typically designated as thefunction {overscore (x)} (λ), primarily comprises the responsivity ofthe red sensitive cones of the human retina. The blue sensitive cones'responsivity is, suitably scaled, also included in this firsttristimulus curve (see FIG. 1b). Curve 2 is, to a good approximation, asummation of the green and red cones' responsivity curves and isdesignated as the function {overscore (y)} (λ) and actually correspondsto the overall spectral sensitivity of the eye. Curve 3 essentiallycomprises the blue cones' spectral sensitivity characteristic {overscore(z)} (λ). It will be clear from these curves that the {overscore (x)}(λ) curve has a subsidiary maximum in the blue region of the visiblespectrum. A colour stimulus to the human visual system may beconveniently expressed as three values, the so-called tristimulus values(X, Y and Z), each of which involves an integral over the visiblespectrum of the spectral power distribution reaching the retinal conesconvolved with the respective tristimulus curve. For example:—

X=∫_(λ)P(λ){overscore (x)}(λ)dλ

[0040] Two further sets of curves are shown in FIG. 1a. One of thesecomprises dashed lines 4 and 5. Line 4 represents, following somenormalisation, the ratio between {overscore (z)} (λ) and the root meansquares of {overscore (x)} (λ) and {overscore (y)} (λ) and line 5represents, on the same basis, the ratio between {overscore (y)} (λ) andthe root mean squares of {overscore (x)} (λ) and {overscore (z)} (λ).

[0041] The objective in calculating these merit functions is to findthose points within the visible spectrum where the effect of theresultant stimulus of the human visual system is substantially expressedas a change to one of the tristimulus values, with the change to theother two being minimised relative thereto. What the two curves show isthat, for a maximum change to Z relative to X and Y, stimulation of thehuman visual system at a wavelength of around 470 nm should be used andthat, for maximum change of Y relative to X and Z, stimulation of thehuman visual system at a wavelength of around 520 nm is most effective.The purpose of the merit function is to find the optimal wavelength formaximising Y, relative to X and Z. Its value peaks near 520 nm, anddrops to half its maximum at approximately 510 nm and also at 540 nm. Achoice of wavelength within this range would be acceptable, though, forbest results, a wavelength between 520 nm and 530 nm should be chosen.There is no clear choice for X, but a wavelength of around 640 nm isfound to achieve good red saturation without too much loss of overallsensitivity.

[0042] It is an objective of this invention to provide a means forcontrolling the colour stimulation of the human visual system, so thatan optimum ratio of X, Y and Z values can be established. When this isachieved, the visual or related disability and/or symptom of thesubject, experienced under normal illumination, can be substantiallyalleviated. It will be clear that a combination of controllablenarrow-band light sources, located respectively at substantially 470 nm,520 nm and, say, 640 nm, will readily achieve this goal. All of thesewavelengths are substantially achieved with commercially availableLED's, the bandwidths of which typically vary from 17 nm to 47 nm.Typical examples of such emitted spectra are shown in FIG. 1a as curve6, for Z, peaking at 470 nm (defined as blue herein), curve 7, for Y,peaking at 524 nm (defined as green herein) and curve 8, for X, peakingat around 640 nm in the red portion of the spectrum. The red wavelengthis not as critical as the other two, for the reasons stated above.

[0043] By combining the light from the three different types of LED, asspecified above, a wide range of colours can be achieved. A lampcomprising one or more of each type of LED, arranged in a variety ofdifferent ways, in which each group of a specific colour is controlledby an adjustable signal, can be used to optimise the illumination for agiven subject carrying out a specific task, such as reading or writing.For example, a person who suffers from dyslexia may have a readingdifficulty significantly alleviated by the partial or complete exclusionof the red illumination, in effect, by reducing the stimulation of thered sensitive cones.

[0044] Embodiments of the current invention use a multi-colour lightemitting diode (LED) array, operated within an optical assembly so thatcolours can be mixed to create the optimum lighting for any patient. Anarray of different coloured LEDs, typically red, green and blue, inaccordance with the principles outlined above are operated eitherindividually or together, so that it is possible to select singleprimary colours or combine the various LEDs to give different hues andilluminance. The primary advantage with this type of lighting being thatit can be used for both reading and writing.

[0045] In practice, each LED type (red green or blue) has its ownchromaticity co-ordinates and the differences between that of one typeand of the other two determine the range of colours that can be achievedby appropriately combining their outputs.

[0046] The table below sets out typical values of x, y and z (in which zis defined as 1-x-y) for each of the three LED types x y z Red 0.7060.294 0.000 Green 0.159 0.717 0.124 Blue 0.129 0.071 0.800

[0047] A method, well established in the prior art, for depicting aparticular colour within a continuum of possibilities is to representthis as a point on a chromaticity diagram of x against y. In such adiagram (see FIG. 5, which depicts the CIE 1931 chromaticity diagram),all possible colours fall within a defined and closed locus of points51. In practice, each LED will have an xy co-ordinate, falling on orwithin this boundary, and the colour co-ordinates 52, 53 and 54 of eachof the three types of LED, between them, define a triangle 55 withinthis complete colour space. The larger this triangle, the greater therange of colours which can be produced by varying the contributions tothe illuminant from each of the LED types. For each target value of xand y, there will be a defined output requirement from each type of LED.It may be shown that, after inverting the matrix comprised of the threecolour co-ordinates (each having three terms) of the red, green and blueLED's provided above and after applying suitable compensation factors toeach drive of the LED types, amongst other things, to compensate fortheir spectral distributions and quantum efficiencies, a 3×3 matrix maybe constructed, which defines the required demand to apply to each ofthe primary sources in order to obtain a specific point (x, y, z) incolour space. This matrix is of the general form and is used asfollows:— $\begin{bmatrix}{Red\_ demand} \\{Green\_ demand} \\{Blue\_ demand}\end{bmatrix} = {\begin{bmatrix}1.095 & {- 0.215} & {- 0.158} \\{- 0.634} & 1.543 & {- 0.033} \\0.063 & {- 0.151} & 0.796\end{bmatrix} \times \begin{bmatrix}x \\y \\z\end{bmatrix}}$

[0048] What the above three relationships define is that, in thisparticular example and for a target white illuminant (x=0.333, y=0.333,z=0.333) to be provided, relative demands of 0.241 from the red source,0.289 from the green source and 0.236 from the blue source are required.If the chromaticity co-ordinates of the red source (0.706, 0.294, 0) areapplied to the right hand side of the above equality then, as expected,the only demand required is that of the red source. The three LED types12, 13 and 14 have chromaticity co-ordinates, depicted in FIG. 5 aspoints 52, 53 and 54. As stated above, theses define a triangle 55within the closed locus of points 51 in the chromaticity diagram whichrepresents the continuum of all colours. If chromaticity co-ordinateswhich fall outside this triangle are applied to the right hand side ofthe above equality, a negative demand from at least one of the primarysources would be indicated. This is not possible and consequently thetriangle defines those colours which the colour selectable lamp of thisinvention can typically provide. It is possible, within the scope ofthis invention, to introduce additional narrowband light sources, suchas, for example, a narrow band source having its peak at 505 nM andhaving colour co-ordinates (x, y, z) of (0.004, 0.655,0.341). This isshown as point 56 in FIG. 5. By defining a second matrix, we maycalculate the demands required from the original green and blue sourcestogether with that from this new blue-green source, in order to accessthat portion of colour space bounded by the triangle defined by points53, 54 and 56.

[0049] In practice the narrow-band sources used in preferred embodimentsof this invention and their particular position in colour space providea very large gamut of possible colours. A colour selectable lampconstructed in accordance with this invention allows much greaterflexibility than that of systems which employ subtractive broadbandfilters to control the colour of the illuminant and provides theopportunity to better taylor the illuminant to each user. This couldhave important applications in the office and school environment whereambient lighting limitations contribute to reading and writing problemsfor some individuals.

[0050] Turning to FIG. 2, this shows diagrammatically how a number ofcomponents may be combined in accordance with the principles of theinvention to form a colour controllable light source.

[0051] A lamp 11 comprises an array of LED's. The array includes redemitters 12, having an emission spectrum peaking at 640 nm, greenemitters 13, having an emission spectrum peaking at 524 nm, and blueemitters 14, having an emission spectrum peaking at 470 nm. The LED'sare distributed in such a manner that the field illuminated by each typeat a reading surface 15 is approximately the same. In order to ensurethat there are no substantial differences in the mix of colours at anygiven point on the reading surface, a diffuser 16 is placed in the pathof the emitted light. This diffuser may take several different forms. Alenticular screen or microlens array is found to be effective, as wellas other kinds of efficient light scattering media. For example, amaterial comprising changes of refractive index over short distances canbe very effective.

[0052] The effect of distributing the individual LED's in an evenmanner, together with the action of the diffuser 16, is to provide avery even mix of light at the reading surface 15. In order to extend theeffective area of illumination, a divergent lens assembly 18 can be veryuseful. Although this is shown as a conventional meniscus lens, acompact equivalent, such as a fresnel lens may also be used.

[0053] A control unit, typically a microprocessor, 19 receives a numberof different inputs, prior to driving each group of LED's via outputs 20for blue, 21 for green and 22 for red. At its simplest level, variableresistors 23, 24 and 25 are used to set the light output from the red,green and blue LED's respectively. The components identified, thus far,comprise a colour controllable lamp. This can be used by a subject toselect a particular combination of red, green and blue illuminants,which is optimal for his or her reading or writing performance.

[0054] In practice, a more sophisticated version of such a lamp wouldadapt the light output demanded from the LED array to take account ofthe ambient conditions. In FIG. 2 a lens 26 forms an image on thereceiving surface 27 of a camera 28. This may be a CCD or otherphoto-detector array, behind a colour filter array. Using knownprinciples, the video signal from the CCD can be analysed to provide areading of the level of illumination at surface 15, in addition to itscolour mix. There will be a specific matrix, which will allow themeasure of light passing through each component of the camera's colourfilter array to be translated into a red, green and blue LED lightcombination. Some of this will be contributed by the ambient lightimpinging on surface 15. The output, required from each type of LED, isadjusted by control unit 19, accordingly. As a consequence of the use ofcamera 28 to monitor the illumination of surface 15 the resulting systemwill also be stabilised against other variations, such as changes in theefficiency of the optics or LED's.

[0055] The apparatus of FIG. 2 can be very useful as a diagnostic tool,particularly when used in conjunction with a computer, shown as block29. Amongst other things, the computer can be used to store the selectedtint of the illumination at surface 15, when this has been optimised orat least substantially improved for the subject.

[0056] Turning to FIG. 3, this outlines, in summary form, a methodologyin accordance with the invention for establishing the optimalillumination for a specific subject, such as, for example, a personsuffering from visual dyslexia.

[0057] The first step in the procedure is to determine the bestillumination conditions for a variety of different reading tests. Thisis done by illuminating the reading material at surface 15 of FIG. 2with one of the illuminants. This is increased in brightness, until thesubject is satisfied that the optimal brightness has been found. It maybe necessary to pass through the optimum and to reduce the brightnessslightly to establish that setting. This step is repeated for each ofthe illuminants (LED groups), separately. It is quite possible that theoptimum level for the red illumination may be at 50% of maximum, for aparticular subject, whereas the green and blue illuminants would bequite acceptable at their maximum levels. The particular settings foreach illuminant will be highly subject dependent. Step 2 is to recordthe optimum level (or the level at which the subject's performanceimproves) for each illuminant, either directly from the controls ortransferred automatically to a computer.

[0058] Once the individual optima have been established, the recordedlevels of each primary illuminant are combined in Step 3 of theprocedure. Step 4 is to fine tune this mixture by making smalladjustments to each primary (red, green and blue), in small steps, untilan optimum or at least improved mix is established for the subject. Thestep changes would be made in both directions, decreasing or increasingthe particular illuminant, and establishing whether there is animprovement or otherwise in the subject's performance. By iteration ofSteps 3 and 4, the best combination is found.

[0059] One of the key objectives of this invention is to use thearrangement of FIG. 2 as a diagnostic tool, in order to arrive at animproved or even optimal formulation for the filters to be provided forthe lenses of spectacles or contact lenses to be worn by the subject.The colour of the light reaching the subject's eyes is recorded by thesystem of FIG. 2 and stored in computer 29. This record will typicallycontain information about the settings of the LED sources and, if any,the colour and level of the ambient illumination at the time that themeasurements were made. By prior knowledge or use of colour camera 28,any colouration of the reading surface 15 may also be accommodated.

[0060] In practice there will be a finite selection of filterformulations available. A typical filter characteristic is shown in FIG.4. Curve 41 represents the percentage transmission of a red absorbing(blue tinted) filter as a function T(λ) of the wavelength λ of the lightincident upon it. Our interest is in knowing what the response at theretina of each eye will be for each of the cones when the subject viewsmaterial through this filter. In order to calculate this we mustmultiply each of the tristimulus curves at every wavelength with thespectral distribution of the light arriving at the retina and integratethis result over the visible spectrum. The result will be one of thetristimulus values for the particular tint, as defined by the CIE 1931chromaticity diagram (as shown in FIG. 5). It will comprise a number ofcomponents, including the following:—

[0061] 1) the spectrum of the illumination which the subject will usewhen reading or writing (This could be daylight or light from a tungstenor fluorescent lamp and each will have a different spectrum),

[0062] 2) the background reflectance spectrum of the material being readand

[0063] 3) the relevant tristimulus curve.

[0064] For the response corresponding to each of the tristimulus valuesthe integral required will be of the form${X = {\int_{380\quad {nm}}^{780\quad {nm}}{{I(\lambda)}{T(\lambda)}{R(\lambda)}{\overset{\_}{x}(\lambda)}{\lambda}}}},$

[0065] Where I(λ) is the illumination spectrum, T(λ) is the filter'stransmission spectrum, R(λ) is the illuminated substrate's reflectancespectrum and {overscore (x)} (λ) is the relevant tristimulus curve,shown, suitably normalised as curve 1 in FIG. 1a. Two further integralswould be calculated for the Y and Z tristimulus values.

[0066] It will be clear to those versed in the art that the sametristimulus values can be achieved with a different illuminationspectrum and, in principle, without the use of the interveningtransmission filter. Indeed, where the illumination spectrum iscomprised of the combination of the three primary illuminants providedby the red, green and blue LED's of FIG. 2, this spectrum will havethree well-defined peaks. As already explained, by reference to FIG. 1aand FIG. 1b, each of these peaks will have a particularly significantinfluence on only one of the tristimulus values.

[0067] It is a further objective of this invention to simulate theeffect of any particular filter by providing illumination whichsimulates the effect on the visual system that would result from the useof that filter under the expected lighting conditions. Thus the LEDoutputs, with the reflectance characteristics of the reading surface 15in FIG. 2 being taken into account, must be adjusted to simulate thatpart of the function under the integral above represented byI(λ)T(λ)R(λ). In effect, I(λ)T(λ) will be replaced by the followingexpression:—

E(λ)=rR(λ)+gG(λ)+bB(λ),

[0068] where r, g and b represent the components of each of the primaryilluminants and R(λ), G(λ) and B(λ) are the respective spectral powerdistributions of these, as shown in FIG. 1a as curves 8, 7 and 6respectively.

[0069] For every choice of filter characteristic available there will bevalues of r, g and b which will simulate the effect for the subjectunder a particular selection of lighting. Having established an optimaltristimulus value for the subject by using the procedure of FIG. 3, abest choice of tint may be selected or formulated. A database of allstandard filters may be held on computer 29, in order to provide aconvenient method for prescribing an available choice of filter. Theprecise effect of that filter being available for the subject toexperience by simulation using the apparatus of FIG. 2

[0070] It follows from this that the apparatus of FIG. 2 may be used todetermine the relative colour response of an individual's eye. In thiscase a surface of known colour reflectance is made to look white byadjusting r, g and b values above. The expression describing this is:

CC[surface(λ)*(E _(r)(λ)*rR(λ)+E _(g)(λ)*gG(λ)+E _(b)(λ)*bB(λ))]=CC _(p)

[0071] where CC[f (λ)] is the colour co-ordinate transformation of aspectrum, CC_(p) is the perceived white colour response and E_(r)(λ),E_(g)(λ) and E_(b)(λ) are the eye responses. For a known surface andinstrument settings and a normal eye response then the perceived whitecolour will correspond with the actual colour co-ordinates of white withCC_(p)=[0.33,0.33,0.33].

[0072] For an eye with a different colour response CC_(p) will be at adifferent position in colour space and the vector between this positionand nominal white will be a measurement of relative colour response ofthe eye.

[0073] By further reference to FIG. 1a it also follows that, in order toreduce the X tristimulus value to a minimum, a light source with itsenergy concentrated at around 505 nm is required. Such a facility mayprove particularly useful in circumstances where the function of thelamp is a diagnostic one and a complete absence of the X stimulus isdesired. Its provision, as illustrated earlier herein, will alsoincrease the range of tints which can be simulated by apparatusconstructed in accordance with the invention.

[0074] Although the embodiment of FIG. 2 incorporates a divergent lensto spread the illumination over the desired area, this is not anessential component for the operation of the lamp, as the combination ofa diffuser and suitably positioned LED's can be chosen to illuminate anyspecific area. Whilst the embodiments illustrated herein utilise LED'swith relatively narrow-band emission spectra, other devices such aslaser sources may be used as alternative illuminants. Furthermore,whereas a camera 28 is employed to analyse the colour of theillumination of surface 15, this could, in practice, be replaced by aseries of photodiodes receiving light from this surface through suitablecolour filters.

[0075] An alternative embodiment of the invention is illustrated in FIG.5. This is similar to the embodiment of FIG. 2, but, instead of a CCDcamera to view the light scattered from reading surface 15, a photocell30, having precisely known spectral sensitivity, is positioned behind asmall aperture 31 in surface 15 at which material to be viewed under acolour controlled illuminant would, in normal use, be placed. A diffuser32 is placed immediately in front of photocell 30 to ensure that itresponds uniformly to light from lamp 11, regardless of its point oforigin at the lamp. Another optical arrangement to achieve this endresult would comprise a lens (not shown) positioned between aperture 31and photocell 30 and arranged to image the lamp onto the photosensitivearea of photocell 30. The function of photocell 30 is two-fold. It isused within an automated calibration procedure to adjust the respectivedrive currents to the red 12, green 13 and blue 14 LED's, in order toprovide the correct balance for a white illuminant. Each LED type isactivated in sequence and the power adjusted to ensure that the expectedresponse, which can be calculated from the known spectral output of theLED and the corresponding spectral sensitivity of photocell 30, isreceived by the latter. Once the LED's have been balanced in this way,they may be used in conjunction with photocell 30 to test thetransmission characteristic at three points of the spectrum of any lens(shown in broken line format as item 33 in FIG. 5) which has beenformulated using a known filter material. For a given filter materialthe ratios of the three responses will be known and the density of thefilter will be calculable. The combination of the selectable LED's andknown photocell characteristic, enables a precise validation oftransmission characteristics of lens 33 to be carried out.

[0076] An embodiment of the invention which includes temperaturecompensation to improve precision is illustrated in FIG. 7. Atemperature sensor 34 is included and is attached to the assembly oflamp 11, which incorporates the LED's. The temperature of this assemblyis relayed via line 35 to microprocessor 19. It has been establishedthat the quantum efficiency of an LED typically changes as a function ofits operating temperature and some loss of light output may be expectedas the device's temperature increases. This effect can be effectivelyoffset by adjusting the demand to the LED as a function of temperatureand line 35 provides microprocessor 19 with the necessary means fordoing so.

[0077] It will be clear to those skilled in the art that the manufactureof any tinted lens, which is formulated as a result of a prescriptionderived from the simulation of such lens using apparatus and methodconstructed in accordance with the teachings of this invention, is theintended end product of such simulation and thereby falls within thescope of the invention.

[0078] The invention having been disclosed in connection with theforegoing variations and examples, additional variations will now beapparent to persons skilled in the art. The invention is not intended tobe limited to the variations specifically mentioned, and accordinglyreference should be made to the appended claims rather than theforegoing discussion of preferred examples, to assess the scope of theinvention in which exclusive rights are claimed

1. Apparatus for the assessment of a subject for at least one of aplurality of vision-related physiological defects and pathologicalconditions comprising a plurality of light sources, each of which isarranged to emit a respective spectral component of the visiblespectrum, and control means for selecting a weighted mixture of saidspectral components and controlling said light sources to provideillumination having the selected spectral components, wherein a firstspectral component has its peak at a wavelength which is located between510 nm and 540 nm and contributes predominantly to a respective firsttristimulus value of the light entering an eye of the subject. 2.Apparatus as claimed in claim 1 in which, in use, said illumination isarranged to illuminate a surface for viewing by the subject; saidmixture is an additive combination of the spectral components emitted byat least two of said light sources; and the control means varies theamount of illumination from each of said at least two light sources toimpinge on said surface whereby, in use, a combination of said spectralcomponents is provided to alleviate the symptoms of at least one of saidvisually induced physiological defects and pathological conditions. 3.Apparatus as claimed in claim 1 in which the first spectral componenthas its peak at a wavelength located between 520 nm and 530 nm. 4.Apparatus as claimed in claim 1 in which a second spectral component hasits peak at a wavelength located between 465 nm and 475 nm.
 5. Apparatusas claimed in claim 1 in which a second spectral component has its peakat a wavelength located between 610 nm and 650 nm.
 6. Apparatus asclaimed in claim 1 in which each of said light sources comprises atleast one respective light emitting diode arranged to provide one ofsaid spectral components.
 7. Apparatus as claimed in claim 1 in whicheach of the spectral components has a spectral power distribution havinga width at half height which does not exceed 50 nm.
 8. Apparatus asclaimed in claim 2 in which the illumination from each of the lightsources is diffused prior to impinging on the surface so that therelative intensity of the light impinging at two points spaced on saidsurface is substantially the same for each of said spectral components.9. Apparatus as claimed in claim 1 further comprising means forcomputing the combined effect of at least two of the active illuminationspectra, the ambient illumination spectrum, the reflectance spectrum ofthe target or an illuminated surface, the transmission spectrum of atleast one filter and the transmission spectrum of a surface coating overthe visible spectrum, so that in use, the subject's retinal response maybe predicted and the settings of the active light source and/or theformulation of a filter to be used by the subject may be improved. 10.Apparatus as claimed in claim 1 in which the control means includes amatrix transformation of desired chromaticity co-ordinates of theillumination whereby, in use, the light sources are controlled toprovide the illumination.
 11. Apparatus as claimed in claim 1 in whichthe control means includes a light sensor for calibration of the lightsources.
 12. Apparatus as claimed in claim 1 in which the control meansincludes a temperature sensor whereby the light sources are controlledto provide the illumination substantially without temperaturedependence.
 13. A method for assessing a subject for at least one of aplurality of vision related physiological defects and pathologicalconditions comprising: arranging a plurality of light sources to emitdifferent spectral components within the visible spectrum; assessing thesubject's performance with a series of targets under different levels ofeach of a plurality of illuminants, including individual spectralcomponents or pre-determined ratios thereof; recording the level of eachof said illuminants corresponding to improved performance by thesubject; and combining the levels of each respective illuminant asrecorded in said recording step to provide a resultant additive mix ofsaid illuminants.
 14. The method of claim 13 including the further stepof applying variations to the level of each of the spectral componentsin small steps whilst combined in order to establish the mix of saidilluminants which substantially improves the subject's performance. 15.A method for the simulation of the use of a filter under expectedlighting conditions comprising: defining the tristimulus values of atint which would be observed under the expected lighting conditions by asubject when said filter is used in transmission for viewing a surface,providing a colour controllable lamp including narrowband coloured lightsources, selecting the level of illumination provided by each lightsource and controlling the colour controllable lamp to illuminate thesurface for viewing by the subject so that, in use, the definedtristimulus values are observed by the subject
 16. The method of claim15 further comprising the step of simulating a range of pre-formulatedfilters and lighting conditions, whereby the subject can select one ormore of said pre-formulated filters for use under said lightingconditions.
 17. The method of claim 15 which includes the further stepof formulating and/or selecting the filter to improve the subject'sperformance.
 18. The method of claim 16 which includes the further stepof formulating and/or selecting one of said preformulated filters toimprove the subject's performance.
 19. A method as claimed in claim 15applied to the formulation of any one of filters and anti-reflectioncoatings for spectacles, contact lenses, coloured overlays and any othertinted material through which the subject may view the surface a purposeof which is to alleviate problems caused by colour-related disorders ofthe human visual system.
 20. An article formulated by the method ofclaim
 19. 21. A method for alleviating the symptoms of any one of aplurality of vision related physiological defects and pathologicalconditions suffered by a subject which comprises: arranging a pluralityof light sources to emit different spectral components within thevisible spectrum, controlling the weight of each of said spectralcomponents to provide illumination, arranging at least two of the lightsources to additively illuminate a surface for viewing by the subject;varying the amount of illumination from each of said at least two lightsources to impinge on said surface; and providing a combination of saidspectral components to alleviate the symptoms of at least one of saidvisually induced physiological defects and pathological conditions. 22.The method of claim 21 in which the physiological defect is visualdyslexia.
 23. The method of claim 21 in which the pathological conditionis visually induced migraine.
 24. The method of claim 21 in which thepathological condition is macular degeneration.